1. Field of the Invention
The present invention relates generally to image forming apparatus and, more particularly, to an image forming apparatus capable of forming an image containing a natural image such as a photographic image and a line drawing such as a character or a figure by utilizing some degrees of resolutions.
2. Description of the Related Art
Conventional apparatus of this kind are typically arranged to produce an image output in which there exist a natural image and a line image by causing the resolution of the natural image to differ from that of a line image by an image-area separating method or in accordance with a command given by a central processing unit (CPU). However, more demands will be placed on an arrangement capable of synthesizing independent images which are represented in different steps of tone representation--a natural image and a line image.
A typical example of such an arrangement is an electrophotographic type of laser beam printer (LBP). The laser beam printer (LBP) is arranged to reproduce a natural image, a line image or the like by converting input image data into two-level signals (PWM signals, dither signals and so on) in accordance with the density represented by the image data.
However, the conventional arrangement has a number of problems. For example, it is necessary to perform the extra step of separating a synthesized image prepared in a host system into image areas within a printer system.
If separation into image areas is to be performed in the printer system, it is necessary to carry out complicated image-area separation in the printer system or image-area separation must be controlled within a limited range by utilizing the CPU of the printer system.
Although it is possible to easily synthesize a natural image and a line image whose numbers of steps of tone representation materially differ from each other, it will be difficult to achieve satisfactory image-area separation with respect to images having close numbers of steps of tone representation.
In addition, general LBPs are arranged to drive laser light with constant power irrespective of the nature of each image. For this reason, it is difficult to faithfully reproduce the nature of various kinds of images (the tone representation of natural images, the sharpness of character images or the like).